The present invention relates to rolled optical film, which shows no light leakage and no luminous streaks due to die lines when being viewed on a liquid crystal display, even without utilizing masking film and a sliding agent.
A liquid crystal display has been commonly utilized as a monitor due to advantages of space saving and energy saving compared to a conventional CRT display. Further, it has been popularized as a TV application. In such a liquid crystal display, utilized are various types of optical film such as polarizer film, phase difference film, anti-reflection film and luminance enhancing film.
Polarizer film is comprised of a polarizer element made of stretched polyvinyl alcohol film on one surface or the both surfaces of which cellulose ester film is accumulated as a protective layer. Further, phase difference film is utilized for the purpose of such as viewing angle enlargement and contrast improvement, and is comprised of film which is made of such as polycarbonate, polymer having an alicyclic structure and cellulose ester, and stretched to be provided with retardation, or a transparent substrate on which a liquid crystal layer is coated. Phase difference film is also referred to as optical compensation film. In such as a liquid crystal panel of a VA mode, phase difference film can be pasted up with a polarizer plate in a roll-to-roll manner when the slow axis of phase difference film is along the width direction, resulting in significant improvement of productivity compared to a conventional batch-wise pasting.
These optical films are required to have no optical defects and to be provided with uniform retardation. In particular, a large image plane and high definition of a monitor and a TV are prevailing to make these required qualities more and more severe.
A manufacturing method of optical film is roughly classified into a solution casting method and a melt casting method.
In a solution casting method, polymer is dissolved in a solvent and the solution is cast on a support, the solvent being evaporated, and is further appropriately stretched to be made into film. Polymer soluble in a solvent can be applicable, and such as norbornene type polymer film and cellulose triacetate film have been commonly utilized with respect to excellent uniformity of layer thickness, however, there were problems of such as requirement of large facilities to dry up the solvent.
Further, optical film is preferably finished not in a sheet form but in a roll form with respect to productivity at the time of such as pasting up with a polarizer plate, however, there has been a case to show light leakage to make an image whitish blurry when an image is displayed on a liquid crystal display in which optical film prepared by a solution casting method, being wound in a roll form as it is, is pasted up with a polarizer plate. Therefore, it has been uneconomical to require subsidiary materials, which are not essentially required, such as to wind up while protecting the surface with a masking film, to incorporate a sliding agent in film or to accumulate a sliding agent on the film surface.
A melt casting method is a method in which polymer is melt with heating to be cast on a support, followed by being cooled to be solidified, and is further appropriately stretched to be made into film. Therefore a melt casting method has an advantage of making the facilities relatively compact since there required no drying of a solvent. However, since the viscosity of melt polymer is significantly higher than that of a polymer solution to make leveling on a support difficult, resulting in a strong defect of a streak form called as a die line in the prepared optical film. When this die line is excessively strong, there has been a problem that luminous streaks due to a die line are observed when the prepared optical film is assembled in a liquid crystal display.
In such a view point, proposed has been optical film in which luminous streaks due to a die line are reduced.
In JP-A 2005-173072 (hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection No.), proposed has been optical film in which die lines formed along the longitudinal direction of optical film is not more than 10 lines per 1 m long in the short direction of optical film.
In JP-A 2005-128360, proposed has been optical film in which a height from the valley bottom to the mountain peak, being adjacent to each other, of a die line formed along the longitudinal direction of optical film is not more than 100 nm in the whole film surface and an inclination of a die line on the film surface is not more than 300 nm/mm in the whole film surface.
In JP-A 2005-55619, proposed has been optical film in which both a depth and a height of a die line formed along the longitudinal direction of optical film are not more than 50 nm and a width thereof is not less than 500 μm.
In JP-A 2000-280315, proposed has been optical film in which the surface roughness is not more than 0.3 μm base on maximum roughness Rt notation.
In any of these examples, the effect is explained in the case of utilizing norbornene type polymer as polymer.
However, when the inventors of this invention wound up cellulose ester film, having die lines in a range of the above described patents, around a core to prepare rolled optical film, which was pasted up on a polarizer plate while being unwound, the polarizer plate accumulated with optical film being cut to be assembled in a liquid crystal display, and an image was displayed thereon; light leakage which makes an image whitish blurred was observed. It has been proved that masking film or a matting agent is still required to solve this problem.
Cellulose film is characterized by being superior in adhesion with a polarizer plate and having suitable moisture permeability when being pasted on a polarizer plate, which is stretched in a wet method, in addition to by having optical uniformity and few optical defects. Therefore, desired has been rolled optical film comprising cellulose ester film in which the above-described various problems have been overcome.
[Patent literature 1] JP-A 2005-173072
[Patent literature 2] JP-A 2005-128360
[Patent literature 3] JP-A 2005-55619
[Patent literature 4] JP-A 2000-280315